The term for the smallest unit of an element is an "atom." Atoms consist of three primary subatomic particles: protons, neutrons, and electrons. Protons and neutrons are located in the atomic nucleus, while electrons orbit around the nucleus in electron shells. Each element on the periodic table is defined by its number of protons, known as the atomic number. Atoms can combine to form molecules, leading to the vast diversity of substances in the universe. Understanding atoms is fundamental to fields like chemistry and physics, as they are the building blocks of all matter.